Exterior surface of a printing machine



FIG. 1 is a top and left side perspective view of an exterior surface of a printing machine showing my new design, the broken lines are for illustrative purposes only and form no part of the claimed design;

FIG. 2 is a front elevational view thereof;

FIG. 3 is a right end view; and,

FIG. 4 is an enlarged front and left side perspective view of an element in FIG. 1. 

The ornamental design for an exterior surface of a printing machine, as shown and described. 